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Oil and Gas: Characteristics of Unconventional Resources

Oil and Gas: Characteristics of Unconventional Resources

Posted on: October 31, 2019

By Usman Ahmed and D. Nathan Meehan

As the shale revolution continues in North America, unconventional resource markets are emerging on every continent. In the next eight to ten years, more than 100,000 wells and one- to two-million hydraulic fracturing stages could be executed, resulting in close to one trillion dollars in industry spending. This growth has prompted professionals experienced in conventional oil and gas exploitation and development to acquire practical knowledge of the unconventional realm.

Oil and gas production from shale reservoirs is growing at a phenomenal rate. The type of shale production being discussed is generally conventional oil or gas produced from relatively deeply buried shales that are produced in a manner roughly similar to conventional wells.

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Shales are the most abundant types and volumes of rocks in sedimentary basins worldwide. Shales are the most abundant sources of hydrocarbons for oil and gas fields and due to their low permeabilities form the hydrocarbon seal for many fields. A conventional oil or gas field needs a source, a reservoir (usually porous sandstones or carbonates such as limestone or dolomite), a trap (such as a structural closure, sealing fault, or pinchout), and a seal. The fact that many shales still contain significant amounts of natural gas is no surprise to drillers and geologists. It is routine to observe natural gas “shows” while drilling through shales, occasionally in significant volumes.

Shales as Resource Plays

A resource play is a relatively large hydrocarbon accumulation that occurs over a broad geological area. In a resource play, the geological risk of encountering the hydrocarbon bearing strata is nearly certain within the play area. A resource play may nonetheless have wide variability in well performance; however it is often the case that such variability cannot easily be predicted in advance or even correlated to conventional measurements (e.g., porosity, thickness). Resource plays have alternately been described as statistical plays, in which an operator must drill a large number of wells and can expect fairly repeatable results if enough wells are drilled.

Shale as Reservoirs?

While shales are known to be the principal sources for conventional hydrocarbon plays as well as seals, shales can also be the reservoir and trap. Most of the gas created in such reservoirs would be thermogenic in origin, although some shales (e.g., the Antrim) have significant quantities of biogenic gas. Gas is stored in shales either as free gas in the pore spaces or adsorbed onto the organic material or surface walls in the shale.


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Explore Unconventional Oil and Gas Resources: Exploitation and Development for vital information on tight oil, deep gas, shallow biogenic gas, heavy oil and/or natural bitumen, shale gas and oil, gas hydrates and coalbed methane among other key oil & gas topics.

This article was cited from Unconventional Oil and Gas Resources: Exploitation and Development, edited by Usman Ahmed and D. Nathan Meehan (2016).